Enormous Gorge Shaped by River's Tectonic Transformation

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The Tsangpo Gorge in Tibet, one of the deepest canyons in the
world, formed when tectonic forces pushed up the earth and
steepened the path of a river that then caused massive erosion, a
new study finds.

The discovery rewrites the
geological history of the region, which some researchers
thought was caused by massive river erosion that triggered
tectonic uplift in the Eastern Himalaya.

"Our observation in the end is relatively simple," said Dirk
Scherler, a geologist at the GFZ German Research Centre for
Geosciences in Potsdam. He and his colleagues said they think an
unknown event caused an
increase in tectonic uplift rates about 3 million years ago.
As the uplift became more pronounced, about 2.5 million years
ago, it dammed the Yarlung Tsangpo River that runs through the
region, preventing it from flowing down the mountains. [ Infographic:
How Tectonics Shaped Yarlung Tsangpo Gorge ]

"So the river either turns into a lake and gives up, or if it's
carrying enough sediment, it can fill in that [area] and keep
spilling over the edge," said Kelin Whipple, a professor of
geomorphology at Arizona State University, who was not involved
in the study.

Once the river dropped enough sediment and made it over the
natural dam, it quickly flowed down the mountain. At that point,
the mountain was steeper because of the increased tectonic
uplift, causing the river to run faster and leading to
vast erosion in the gorge, the researchers said.

Yet, the research team needed evidence of the ancient sediment to
support their idea. The year before, civil engineers from the
China Earthquake Administration had gathered core samples after
drilling at five locations along the Yarlung Tsangpo River. One
researcher visited the California Institute of Technology in
Pasadena, and shared the core samples with Scherler, who was
completing his postdoctoral research in geology.

Scherler and his colleagues examined the drill-core samples —
some retrieved from up to 0.6 miles (1 kilometer) deep — to see
whether the area in question had a sediment deposit. "And sure
enough, there was," said Whipple, who wrote an opinion piece,
published today (Nov. 20), about the study for the journal
Science. "And they show it very nicely in their study."

The core samples contained sand, rounded gravel and larger rocks
cemented together, a mix indicative of sediment from a flowing
river, the researchers said. About 2,600 feet (800 meters) below
the surface, the samples contained bedrock, showing that the
sediment had filled an ancient canyon.

The researchers studied the sediments at the bottom of the core
samples — the
earliest sediment layers — and measured two isotopes:
beryllium-10 and aluminum-26. These isotopes are made when
sediment is exposed to cosmic rays, high-energy radiation from
space. The isotopes decay at different rates once the sediment is
buried, and the river began dropping sediment about 2.5 million
years ago, the researchers said.

Different ideas

Until now, many researchers viewed part of the Tsangpo Gorge,
called the Namche Barwa massif, as a poster child of how rivers
may influence
tectonics.

Perhaps the Yarlung Tsangpo River collided with the Brahmaputra
River, which are now connected, the model suggested. The
collision may have diverted the Yarlung Tsangpo River, and caused
it to cut down the mountain, quickly eroding it.

As the rock eroded, it would have become lighter, making it
easier for the tectonic forces below to push the rock up in a
so-called "tectonic aneurysm," which would have made the
mountains steeper, Whipple said.

"In response to that very rapid erosion, you're thinning and
weakening the crust as it's getting hotter in response to the
erosion, bringing hot rocks closer to the surface," Whipple said.
"And that induces a response where the rocks actually start
flowing toward that spot, and cause a bunch of uplift."

Another idea suggests that glaciers
interacting with the Yarlung Tsangpo River cut out the gorge
during the various ice ages, Scherler said.

It's still possible that a tectonic aneurysm took place in the
gorge before the unexplained uplift, but not 2.5 million years
ago, the researchers said.

"Here, erosion has not triggered tectonics, but the river has
responded to the tectonics," Scherler said.